fix: move prettier in dev dependencies (#55)

prettier should not be in dependencies :)

Author: christophehurpeau

package.json

@@ -22,8 +22,7 @@
   "license": "MIT",
   "dependencies": {
     "@octokit/request-error": "^2.0.2",
-    "@octokit/types": "^6.0.3",
-    "prettier": "^2.0.5"
+    "@octokit/types": "^6.0.3"
   },
   "devDependencies": {
     "@octokit/core": "^3.1.1",
@@ -36,6 +35,7 @@
     "@types/jest": "^26.0.0",
     "fetch-mock": "^9.10.7",
     "jest": "^25.1.0",
+    "prettier": "^2.0.5",
     "semantic-release": "^17.0.0",
     "ts-jest": "^25.1.0",
     "typescript": "^3.7.2"